Heat the oil in a large skillet and saute the onions over medium-high heat, frequently stirring, until they become translucent and begin to brown.
Add the minced garlic, diced celery and carrot and cook for another minute or so.
Add the minced meat, salt and pepper and sweet paprika. Stir fry for 15 minutes, breaking up clumps.
Once the meat stuffing is done cooking, move from the heat and let it cool down. Mix in cooked rice. Taste for salt and pepper and adjust to taste.
While the stuffing is cooling down, rinse the peppers, cut off the tops with stems, and remove seeds and membranes.
Fill each pepper generously with the stuffing mixture.
Transfer peppers to a slow cooker greased with some olive oil. Cover and cook on high for 2-3 hours or low for 4-6 hours, or until the peppers are tender to your liking.
To serve, very gently remove the peppers with a large slotted spoon and place on serving plates. Serve drizzled with pan juices. You can top the peppers with yogurt or sour cream and chopped parsley. Creamy mashed potatoes will make a great side dish if desired.
I used 4 very large bell peppers for the amount of specified stuffing. If your peppers are smaller, you may need to use 6-8 of them.